1. Field of the Invention
The present invention relates, in general, to processes for recycling thermoplastic material.
2. Description of the Art
Plastic materials of all types have found many uses in a large number of consumer products. However, such widespread use of plastics has created a problem relating to the disposal of plastic consumer products after their useful life. In addition to used consumer products, waste parts, bad parts, scrap, runners, sprues, purgings, etc., generated from plastic manufacturing processes must also be disposed of.
Currently, plastic materials are incinerated, buried in a land fill, provided with biodegrading properties or, and only currently to a small extent, recycled. Incineration obviously creates a potential environmental problem and is particularly disfavored. Available land fills are becoming crowded and less available, particularly as the use of plastic increases and the amount of land available for land fills decreases. With regard to biodegradable plastics, not a significantly high percentage of such plastics are biodegradable and, even if biodegradable, are costly.
Recycling then becomes the most promising method of "disposing" of plastic materials in used consumer products as well as plastic materials generated during the manufacture of consumer products. The oldest and most used method of recycling involves use of relatively homogeneous recycle. The process disclosed in U.S. Pat. No. 3,567,815 uses high bulk density polystyrene blended with reclaimed low bulk density polystyrene to provide an extruded polystyrene sheet with a uniform thickness. U.S. Pat. No. 3,976,730 defines a special process for adding virgin polyethylene to ground, low bulk density, reclaimed, polyethylene film to yield an uninterrupted blown film of polyethylene.
In some cases, such as in the process disclosed in U.S. Pat. No. 5,122,398, great effort is directed to producing items out of a single base material. In the '398 process, a bumper is produced out of a combination of a glass reinforced polypropylene bumper support layer with a foamed polypropylene core and an outer layer of rubber modified polypropylene. Since all material is polypropylene, or compatible with polypropylene, the entire bumper can be recycled with the dispersed reinforcing glass fiber and the dispersed rubber particles not detracting from properties of the polypropylene.
A typical recycling process involving plastic materials requires sorting the products according to the different types of plastics contained therein. Sorting is typified by U.S. Pat. No. 5,176,861 which describes a process to recycle sorted polyester items, such as bottles, trays and other containers. This process allows conversion of the sorted consumer waste into usable uniaxially oriented polyester sheet which can be further transformed into a variety of useful items. Processes, such as described in the above '815 and '730 patents, are known for mixing small percentages, typically up to 20% of the same virgin plastic with the low density reground used or scrap plastic material to increase the bulk density so it can then be extruded and reformed into useful products similar to those that could be formed if virgin material of slightly lower molecular weight were used.
Dealing with mixed waste with inherently incompatible materials is much more difficult. In U.S. Pat. No. 5,145,617 a high quality composite roofing membrane trimming waste stream composed of a fiber modified composite of PVC and PET is treated. This process is successful in reclaiming a usable material for applications which have low property requirements, such as roof pads of the type used to provide pathways for workers applying membrane material to roofs, etc. The process disclosed in U.S. Pat. No. 3,687,873 attempts to deal with a variety of mixed waste streams by the addition of chlorinated polyethylene (CPE) as a compatibilizing agent. While properties are improved over that of the blends without the CPE they are still greatly reduced from virgin material. For example, an ABS with a tensile strength of 6004 psi is mixed 50/50 with polystyrene having a tensile strength of 6000. The blend has a tensile strength of 881 psi which is raised to a still low value of 2756 when CPE is added.
The above-described recycling process is applicable only to commodity type plastics, such as polyethylene, polypropylene and styrenes. Such recycling processes are not easily used with engineering plastics, including glass or mineral reinforced plastic or plastics which are modified by use of various additives for a particular purpose, such as materials having a high impact strength, tensile strength, etc. Examples of such engineering plastics include polycarbonate, Nylon, acetal, ABF, and the like.
The use of engineering plastics in automobiles is increasing for a variety of applications, including exterior or interior parts of the vehicle which require painting. Painted engineering plastic parts create additional problems which has heretofore prevented their easy recycling into new products. Often when a painted part has a flawed surface, the flaw may be corrected by repainting. Sometimes as many as four coats of paint will be used before the part is scraped. The presence of paint specks in reclaimed parts gives a flawed appearance and greatly reduces all important physical properties of the plastic material.
Thus, it would be desirable to provide a process for recycling thermoplastic materials which, enables processed plastic materials to be rejuvenated into plastic material having nearly the same key properties and key characteristics as virgin plastic materials. It would also be desirable to provide a process for recycling thermoplastic materials and blends of thermoplastic materials which rejuvenates engineering plastics to a state having major properties nearly equal to the same or other virgin plastic materials. It would also be desirable to provide a process for recycling thermoplastic materials which enables painted plastics to be recycled. It would be particularly desirable to have a process which is capable of removing paint specks from multi-coated painted plastic parts being reclaimed.